Disubstituted dialkoxysilanes having the general structure R.sub.2 Si(OR).sub.2 are included, among other things, as stereomodifiers in catalysts for the production of polypropylenes (PP) (European Patent Application EP-A-No. 0,231,878). Particularly advantageous for that use are disubstituted dialkoxysilanes having the formula R.sup.1 R.sup.2 Si(OR.sup.3).sub.2, and especially those compounds where R.sup.1 and R.sup.2 are branched alkyl chains (EP-A-No. 0,250,229 and DE-A-No. 36 29 932).
It is well known that disubstituted dialkoxysilanes R.sup.a R.sup.b Si(OR.sup.c).sub.2 can be produced through alkylation or arylation of tetraalkoxysilanes and only occasionally produced by reaction of monosubstituted trialkoxysilanes with Grignard reagents (Houben-Weyl, Methods of Organic Chemistry, XIII/5, 180 p).
However, as a rule, the reaction products are produced as mixtures of diorganodialkoxysilanes with monoorganotrialkoxysilanesand/or triorganomonoalkoxysilanes, so that the isolation of the desired diorganodialkoxysilane requires a separation step. Hence, one must accept a relatively low product yield with a concomitant increase in production costs (Z. Lasocki, Bull. Acad. Polon. Sci., Ser. Sci. Chim. 12 (5), 281-287 (1964)).
It has now surprisingly been found that by reacting monoorganotrialkoxysilanes R.sup.1 Si(OR.sup.3).sub.3 or tetraalkoxysilanes Si(OR.sup.3).sub.4 with Grignard reagents R.sup.2 MgX in appropriate solvents certain diorganodialkoxysilanes R.sup.1 R.sup.2 Si(OR.sup.3).sub.2, depending on the structure of the R.sup.1 and R.sup.2 groups, are readily produced with a high degree of selectivity and in high yields. The high selectivity and yield obtained by the methods of the present invention provides a means to rapidly and cost efficiently produce diorganodialkoxysilanes. Selective production of diorganodialkoxysilanes depends on at least one of R.sup.1 and R.sup.2, and preferably both, being a branched or a cyclic alkyl group. This dependence was heretofore unknown to the skilled artisan.